1. Field of the Invention
The present invention relates to an active matrix color display device in which pixels are arranged in matrix form and, more specifically, to a technique of inhibiting striped unevenness in luminance profile that appears in color display devices having delta-arranged pixels, for example.
2. Description of the Related Art
An example of a conventional active matrix color display device will be described briefly with reference to FIG. 1. As shown in FIG. 1, a color display device 0 has scanning lines 3 extending in the row direction, signal lines 2 extending in the column direction, and pixels R, G, and B of the three primary colors that are disposed in the vicinity of the crossing portions of the scanning lines 3 and the signal lines 2. The pixels R, G, and B are arranged at a predetermined pitch in the row direction. The color display device 0 also has horizontal switches HSW1, HSW2, . . . , HSWn-1, and HSWn each of which is connected to the ends of three of the signal lines 2. Each horizontal switch HSW selects the associated three signal lines 2 at the same time and writes three image signals VR, VG, and VB of the three primary colors to the corresponding three pixels R, G, and B.
FIG. 2 is a schematic diagram in which attention is paid to one pixel G. Signal lines to which image signals of green and blue are to be allocated are disposed on both sides of pixel G. A signal line to which an image signal of red is to be allocated is disposed further away. Pixel G is connected, via a switch, to the signal line to which green is allocated. An image signal of green is written to pixel G when the switch is closed, and the image signal thus written is held by opening the switch thereafter. Inactive matrix color display devices, because of their structure, there necessarily exist portions where pixel G and the signal lines on both sides overlap with each other with an insulating film interposed in between and parasitic capacitances are formed there. Therefore, even in a state that the switch is open, image-signal-related noise enters pixel G through parasitic capacitance coupling and causes a luminance variation. In particular, in the case of the structure shown in FIG. 1, while a large amount of image-signal-related noise enters pixel G from the signal lines to which green and blue are allocated, almost no image-signal-related noise enters pixel G from the signal line to which red is allocated because it is distant from pixel G.
As shown in FIG. 3, noise relating to an image signal of red (hereinafter referred to as noise R) and noise relating to an image signal of green (hereinafter referred to as noise G) enter pixel R from the signal lines on both sides. Noise G and noise relating to an image signal of blue (hereinafter referred to as noise B) enter pixel G from the signal lines on both sides. Noise B and noise R enter pixel B from the signal lines on both sides. In this manner, each pixel receives large amounts of noise corresponding to two of the three primary colors (red, green, and blue) and a small amount of noise corresponding to one of the three primary colors.
FIG. 4 is a schematic diagram showing an example of a conventional active matrix display device having what is called delta-arranged pixels. In the delta arrangement, three pixels of red, green, and blue belonging to two rows adjacent to each other (in FIG. 4, an odd-number row and an even-numbered row) are located at the apices of a triangle. These pixels are called a pixel trio, and one trio is indicated representatively by circling characters R, G, and B in FIG. 4. The resolution of the delta arrangement of FIG. 4 is about 1.5 times that of the ordinary pixel arrangement of FIG. 3.
In the delta arrangement, colors allocated to signal lines adjacent to one pixel on an odd-numbered row are different from those allocated to signal lines adjacent to one pixel of the same color on an even-numbered row. As a result, the amount of noise entering a pixel on an odd-numbered row is different from that of noise entering a pixel of the same color on an even-numbered row. Now, attention is paid to pixel G, for example. Pixel G on an odd-numbered row receives noise R and noise G but receives almost no noise B. Pixel G on an even-numbered row receives noise G and noise B but receives almost no noise R. Therefore, in a case where an image signal corresponding to green (hereinafter referred to as an image signal G) has an intermediate potential (gray level), an image signal corresponding to red (hereinafter referred to as an image signal R) has a high potential (black level), and an image signal corresponding to blue (hereinafter referred to as an image signal B) has a low potential (white level), in a normally white mode the sum of amounts of noise entering pixel G on an odd-numbered row is large and hence pixel G becomes darker. On the other hand, the total amount of noise entering pixel G on an even-numbered row is small and hence pixel G becomes brighter. Conversely, in a case where a pixel signal G has a gray-level potential, an image signal R has a white-level potential, and an image signal B has a black-level potential, the total amount of noise entering pixel G on an odd-numbered row is small and hence pixel G becomes brighter. On the other hand, the total amount of noise entering pixel G on an even-numbered row is large and hence pixel G becomes darker.
As described above, in the delta-arranged structure, unevenness in luminance profile that varies alternately with rows (hereinafter referred to as horizontal stripes) is caused by the parasitic capacitance coupling between each pixel and the signal lines adjacent thereto. This is a problem to be solved.
An object of the present invention is to inhibit unevenness in luminance profile in a color display device.
To attain the above object, a color display device comprises, as the basic configuration, signal lines, scanning lines, pixels, and color filters. The signal lines are arranged approximately in columns and are to be supplied with image signals separately for red, green, and blue. The scanning lines are arranged approximately in rows so as to cross the signal lines, and are to be supplied with scanning signals. The pixels are disposed in the vicinity of the crossing portions of the signal lines and the scanning lines, and are to be subjected to writing of image signals when selected according to the scanning signals. The color filters allocate red, green, and blue to pixels so as to associate the pixels with signal lines of red, green, and blue, respectively. As an important feature, parasitic capacitances are so formed that the amounts of image-signal-related noise entering each of the pixels from the three closest signal lines of red, green, and blue through parasitic capacitance coupling are approximately equal to each other.
More specifically, the pixels have a delta arrangement in which three pixels of red, green, and blue belonging to two rows adjacent to each other are located at the apices of a triangle, and the parasitic capacitances are so formed that the amounts of noise entering each of the pixels from the three closest signal lines of red, green, and blue that are two signal lines adjacent to the pixel and located on both sides and one signal line further away are approximately equal to each other.
Each pixel comprises a pixel electrode, a switching element for driving the pixel electrode, and a light shield film that has the same potential as the pixel electrode and overlaps with the three signal lines of red, green, and blue with an insulating film interposed in between, to thereby form the parasitic capacitances. The light shield film has an extension that overlaps with the signal line further away with the insulating film interposed in between.
According to the invention, the levels of entrance of noise R, noise G, and noise B are made approximately equal to each other. Therefore, even in the delta arrangement, the total amount of noise entering a pixel on an odd-numbered row from the signal lines has almost no difference from that of noise entering a pixel on an even-numbered row, whereby horizontal stripes can be avoided. For example, as shown in FIG. 5, with attention paid to pixel G, not only does it receive large amounts of noise G and noise B as in the conventional case but also it is adapted to receive a large amount of noise R.